276 VERA DANCHAKOFF 



may be advantageous to demonstrate them by a study of a 

 fully developed spleen, and then to attempt to find out whether 

 the gradual development of the peculiarities influence the his- 

 togenesis of the hematopoietic tissue. A distinctive feature in 

 the spleen structure is given by its special vascularization. The 

 regions with veinous and arterial vascularization, though they 

 penetrate each other, remain nevertheless independent, and 

 communicate together only in places, where the white pulpa 

 passes into the red. The most characteristic cell element of the 

 spleen — the small lymphocyte — belongs to the white pulpa and 

 accumulates here in the form of follicles and follicular strings. 

 The granulocytoblasts, though not numerous in the adult spleen, 

 are chiefly localized in the pulpa. Other ameboid elements, the 

 basophilic large lymphocytes (lymphoid hemacytoblasts), and 

 mononuclear leucocytes, often in the form of macrophages, are 

 common to all the regions of the spleen. The syncytial cell 

 reticulum is also ubiquitous; it forms in the red pulpa wide 

 meshes. In the white pulpa the cells of the reticulum appear 

 denser, and the meshes formed by their ramifications are smaller. 



As a further study of the spleen development will show, the 

 chief characteristic feature, primarily, determining different 

 regions of the spleen as red or white pulpa, consists in the type 

 of vessels by which a region of the spleen is supplied rather than 

 by the presence of certain kind of ameboid cells. The wide 

 veinous capillary net together with sinuses and lacunae forms the 

 red pulpa, the bunches of arteries resolving themselves into a net 

 of narrower branches — belong to the white pulpa. The question, 

 whether the specific differentiation of the ameboid elements 

 depends upon the peculiar vascularization of the spleen may be 

 decided after a study of the spleen development. The chick 

 spleen is a favorable subject for elucidation of this question, 

 for the identification of the vessels is easy in the spleen anlage 

 from the time of their appearance. 



As mentioned above, the development of the spleen in the 

 earliest stages is characterized by its loose mesenchymal structure. 

 The intense cell proliferation leads soon to a transformation 

 of the loose mesenchymal anlage into a denser syncytium. 



